Coulometric timing cells of the type having an erodable conductive metallic anode filament electrode with two terminal connectors extending therefrom to determine whether the filament is ruptured by electrolytic action are known in the art. Examples of this art together with corresponding operating circuits include U.S. Pat. Nos. 3,355,731 issued Jan. 27, 1965, and 3,769,557 issued Oct. 30, 1973.
Also known in the art are two terminal electrolytic timer cells which have only cathode-anode terminals available separated by an electrolyte solution of a relatively high impedance compared to a short circuited metallic wire conductor. This high impedance is varied slightly by subtle changes occurring as metal is plated from the anode to cathode. Such changes are hard to detect and require sensitive amplifiers working within close tolerances.
It is highly desirable in reliable electronic arts to have binary type operations which positively indicate on-off conditions such as closing and opening of a switch. Under such conditions detection of a change such as occurring at the end of a predetermined plating time represented by change of a short circuit to a high impedance condition would present reliable operating conditions not easily disturbed by transient noise conditions.
In general, applications of these cells and circuits have been in automobiles using low voltage d-c power sources such as 12 volt batteries. However, the timing function is applicable for other purposes such as signifying maintenance time after a predetermined number of elapsed running hours of an electric motor operating at 110V or 220V AC. For such applications the prior art has not provided acceptable inexpensive timer circuits operating from high voltage AC without introduction of transformers and power supply circuits. When used in these environments several special problems are encountered when amplifiers are necessary because of sensitivity to high voltage transient signals present. Thus, immunity to high voltage electrical noise is a desirable feature not generally available in prior art devices.
Also in many such applications there is a requirement for very low cost in order to justify use so that simplified operating circuits without necessitating sensitive amplifiers and noise immunity devices are essential.
Furthermore, in general, reliability is decreased in circuits having greater numbers of parts, because of the increased chances for circuit failure.
A still further problem particularly with high voltage -- high impedance circuits is the propensity for failure in the presences of environmental conditions of humidity and dust, or changes in ambient temperature. Also changes in operating voltages tend to cause errors in some types of prior art timing circuits.
It is therefore a general object of this invention to provide improved coulometric timing circuits resolving one or more of the aforesaid deficiencies.
A more specific object of the invention is to provide coulometric timing circuits operable directly from high voltage AC lines without isolation transformers.
Another object of the invention is to provide coulometric timing circuits having fewer parts.
Also, objects of the invention are to provide operation of a coulometric cell with efficient operation at low impedance levels, and under conditions of varying voltage and temperature.
Other objectives, features and advantages of the invention will be found throughout the following specification.